This relates to graphics processing.
It is axiomatic that traffic to memory should be reduced as much as possible in any type of computer architecture, as this saves power and/or provides better performance, or enables other parts in an architecture to exploit more bandwidth.
As such, it is important to compress color data, since this reduces the memory traffic. Multi-sampling anti-aliasing (MSAA) increases the image quality on rendered edges, and it is a common method to increase rendering quality. For 4×MSAA, for example, each tile (e.g., 8×4 pixels) may be divided into 4 planes. Each sample has then 2 index bits (in an index buffer), which indicate which color plane that sample points to. Planes are allocated from low numbers to high numbers, and plane 0 is always allocated. If all samples in a tile point to plane 0, for example, then all index bits are 00.
Compression methods can compress the individual planes, but compression methods do not compress the index buffers. For an 8×4 tile of pixels with four samples per pixel (4×MSAA), the index bits will use 2*4*8*4=256 bits. In some examples, the index bits are using 30% of the memory traffic for color.